Single-level store
Encyclopedia
Single-level storage is a term most often associated with the IBM
System i operating system
, IBM i (formerly known as i5/OS or OS/400), although it was originally introduced in 1962 by the Atlas system at Cambridge. Since it filled an important need, Atlas's storage system was widely copied, first by Multics
in a project shared by MIT
, General Electric
and Bell Labs
. IBM first implemented SLS in 1978 in the System/38
and its Control Program Facility (CPF) operating system, the predecessor to IBM i.
With a single-level storage the entire storage of a computer is thought of as a single two-dimensional plane of addresses, pointing to pages. Pages may be in primary storage (RAM
) or in secondary storage (disk); however, the current location of an address is unimportant to a process. The operating system takes on the responsibility of locating pages and making them available for processing. If a page is in primary storage, it is immediately available. If a page is on disk, a page fault
occurs and the operating system brings the page into primary storage. No explicit I/O to secondary storage is done by processes; instead, reads from secondary storage are done as the result of page faults and writes to secondary storage are done when pages that have been modified since being read from secondary storage into primary storage are written back to their location in secondary storage.
With the i5/OS implementation of single-level storage, page faults are divided into two categories. These are database faults and non-database faults. Database faults occur when a page associated with a relational database object like a table, view or index is not currently in primary storage. Non-database faults occur when any other type of object is not currently in primary storage.
The i5/OS treats all secondary storage as a single pool of data, rather than as a collection of multiple pools (file systems), as is usually done on Unix
- and CP/M
-based systems like BSD
, Linux
and Microsoft Windows
. It intentionally scatters the pages of all objects across all disks so that the objects can be stored and retrieved much more rapidly. As a result, an i5/OS server rarely becomes disk bound. Single-level storage operating systems also allow CPU, memory and disk resources to be freely substituted for each other at run time to smooth out performance bottlenecks.
IBM’s design of the single-level storage was originally conceived in the late 1970s as a way to build a transitional implementation to computers with 100% solid state memory
. The thinking at the time was that disk drives would become obsolete, and would be replaced entirely with some form of solid state memory. i5/OS was designed to be independent of the form of hardware memory used for secondary storage. This has not come to be, however, because while solid state memory has become exponentially cheaper, disk drives have also become similarly cheaper: thus the price ratio in favour of disk drives continues—very much higher capacities than solid state memory, very much slower to access, and much less expensive.
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
System i operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
, IBM i (formerly known as i5/OS or OS/400), although it was originally introduced in 1962 by the Atlas system at Cambridge. Since it filled an important need, Atlas's storage system was widely copied, first by Multics
Multics
Multics was an influential early time-sharing operating system. The project was started in 1964 in Cambridge, Massachusetts...
in a project shared by MIT
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
, General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
and Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
. IBM first implemented SLS in 1978 in the System/38
System/38
The System/38 was a midrange computer server platform manufactured and sold by the IBM Corporation. The system offered a number of innovative features, and was the brainchild of IBM engineer Dr. Frank Soltis...
and its Control Program Facility (CPF) operating system, the predecessor to IBM i.
With a single-level storage the entire storage of a computer is thought of as a single two-dimensional plane of addresses, pointing to pages. Pages may be in primary storage (RAM
Ram
-Animals:*Ram, an uncastrated male sheep*Ram cichlid, a species of freshwater fish endemic to Colombia and Venezuela-Military:*Battering ram*Ramming, a military tactic in which one vehicle runs into another...
) or in secondary storage (disk); however, the current location of an address is unimportant to a process. The operating system takes on the responsibility of locating pages and making them available for processing. If a page is in primary storage, it is immediately available. If a page is on disk, a page fault
Page fault
A page fault is a trap to the software raised by the hardware when a program accesses a page that is mapped in the virtual address space, but not loaded in physical memory. In the typical case the operating system tries to handle the page fault by making the required page accessible at a location...
occurs and the operating system brings the page into primary storage. No explicit I/O to secondary storage is done by processes; instead, reads from secondary storage are done as the result of page faults and writes to secondary storage are done when pages that have been modified since being read from secondary storage into primary storage are written back to their location in secondary storage.
With the i5/OS implementation of single-level storage, page faults are divided into two categories. These are database faults and non-database faults. Database faults occur when a page associated with a relational database object like a table, view or index is not currently in primary storage. Non-database faults occur when any other type of object is not currently in primary storage.
The i5/OS treats all secondary storage as a single pool of data, rather than as a collection of multiple pools (file systems), as is usually done on Unix
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
- and CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
-based systems like BSD
Berkeley Software Distribution
Berkeley Software Distribution is a Unix operating system derivative developed and distributed by the Computer Systems Research Group of the University of California, Berkeley, from 1977 to 1995...
, Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
and Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
. It intentionally scatters the pages of all objects across all disks so that the objects can be stored and retrieved much more rapidly. As a result, an i5/OS server rarely becomes disk bound. Single-level storage operating systems also allow CPU, memory and disk resources to be freely substituted for each other at run time to smooth out performance bottlenecks.
IBM’s design of the single-level storage was originally conceived in the late 1970s as a way to build a transitional implementation to computers with 100% solid state memory
Solid-state drive
A solid-state drive , sometimes called a solid-state disk or electronic disk, is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive...
. The thinking at the time was that disk drives would become obsolete, and would be replaced entirely with some form of solid state memory. i5/OS was designed to be independent of the form of hardware memory used for secondary storage. This has not come to be, however, because while solid state memory has become exponentially cheaper, disk drives have also become similarly cheaper: thus the price ratio in favour of disk drives continues—very much higher capacities than solid state memory, very much slower to access, and much less expensive.
See also
- Atlas (computer)
- System/38System/38The System/38 was a midrange computer server platform manufactured and sold by the IBM Corporation. The system offered a number of innovative features, and was the brainchild of IBM engineer Dr. Frank Soltis...
- IBM i5/OS
- EROSExtremely Reliable Operating SystemEROS is an operating system developed by The EROS Group, LLC., the Johns Hopkins University, and the University of Pennsylvania. Features include automatic data and process persistence, some preliminary real-time support, and capability-based security. EROS is purely a research operating system,...
- MulticsMulticsMultics was an influential early time-sharing operating system. The project was started in 1964 in Cambridge, Massachusetts...
- Virtual memoryVirtual memoryIn computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
- Memory-mapped fileMemory-mapped fileA memory-mapped file is a segment of virtual memory which has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on-disk, but can also be a device, shared memory object, or other resource...